text: The Keggin structure is the best known structural form for heteropoly acids. It is the structural form of α-Keggin anions, which have a general formula of , where X is the heteroatom (most commonly are pentavalent phosphorus P, tetravalent silicon Si, or trivalent boron B), M is the addendum atom (most common are molybdenum Mo and tungsten W), and O represents oxygen. The structure self-assembles in acidic aqueous solution and is a commonly used type of polyoxometalate catalysts.